The present invention relates to a method of forming a tungsten film having an improved resistivity.
Generally, a film of a metal or a metal compound, such as W (tungsten), WSi (tungsten silicide), Ti (titanium), TiN (titanium nitride)or TiSi (titanium silicide), is deposited to form a wiring pattern on a semiconductor wafer, to fill up holes between wiring lines or to form wiring patterns and fill up holes between wiring lines in a semiconductor integrated circuit fabricating process.
Methods of forming such metal thin films are classified into those of H2 reduction system (hydrogen reduction system), those of SiH4 reduction system (Silane reduction system) and those of SiH2Cl2 reduction system (dichlorosilane reduction system). When forming a wiring pattern by the method of SiH2Cl2 reduction system, dichlorosilane gas is used as a reducing gas and a W or a WSi film (tungsten silicide film) is formed at a high temperature on the order of 600xc2x0 C. When forming a wiring pattern by the method of SiH4 reduction system, silane gas is used as a reducing gas and a W or WSi film is formed at a low temperature on the order of 350xc2x0 C.
The method of H2 reduction system is applied mainly to filling up holes in the surface of a wafer, such as holes between wiring lines, uses hydrogen gas as a reducing gas and deposits a W film at a temperature in the range of about 400xc2x0 to about 430xc2x0 C.
All those methods use, for example, WF6 (tungsten hexafluoride). A conventional tungsten film forming method will be explained. A thin Ti/TiN film, for instance, is formed as a barrier metal layer on a surface of a semiconductor wafer before forming a tungsten film. Film forming gases including WF6 gas, SiH4 gas, H2 gas, Ar gas, N2 gas and the like are supplied into a film forming chamber to deposit tungsten seed crystal grains on the surface of the barrier metal layer.
The film forming chamber is evacuated temporarily to a base pressure to remove residual gases from the film forming chamber, and then the Ar gas, H2 gas and N2 gas are supplied into the film forming chamber to set the film forming chamber quickly at a process pressure. Subsequently, WF6 gas is supplied at a predetermined flow rate into the film forming chamber to deposit a tungsten film by hydrogen reduction using H2 gas without using SiH4 gas. Thus, for example, filling up holes and forming a wiring layer are carried out simultaneously.
The development of multilayer semiconductor integrated circuits, and the progressive miniaturization and rise in the level of integration require further reduction of width of lines and diameters of holes. When a wiring pattern is miniaturized, the resistance of wiring lines increases accordingly. Resistivity of wiring lines low enough for conventional design must be reduced further when wiring patterns are miniaturized.
However, it has been difficult to form tungsten films having a satisfactorily low resistivity and meeting new design by the foregoing conventional tungsten film forming method.
A method intended to form a tungsten film having a reduced resistivity to solve the foregoing problems supplies a boron-containing gas such as diborane (B2H6) gas with Ar gas and N2 gas into the film forming chamber to form a tungsten film of tungsten crystal grains of large grain sizes to reduce resistivity. This method, however, is incapable of forming the tungsten film having satisfactorily low resistivity, and the nitrogen-diluted borane gas produces a solid by polymerization in gas supply pipes and the solid clogs the gas supply pipes.
Generally, unnecessary films are deposited in the processing vessel of a film forming system as a film forming process is repeated certain cycles, and the films fall off in particles. Therefore, a cleaning process is carried out at regular or irregular intervals to remove the unnecessary films by supplying a cleaning gas, such as ClF3 gas, into the processing vessel. The cleaning gas remains, though in only a very small amount, in the processing vessel after cleaning and Cl and F atoms contained in ClF3 gas are introduced into the surface of a semiconductor wafer and act as detrimental impurities.
The present invention has been made in view of such problems and has been created to solve those problems effectively. It is therefore an object of the present invention to provide a tungsten film forming method capable of forming a tungsten film having a low resistivity.
The inventors of the present invention made earnest studies of tungsten film forming methods and acquired a knowledge that crystal grains of large grain sizes can be formed by using a gas containing hydrogen and a borane, such as diborane, for forming a tungsten film, and crystal grains of large grain sizes can be formed by carrying out a tungsten film forming process immediately after processing a semiconductor wafer by a boronizing surface treatment process using a boron-containing gas, such as B2H6 gas, and have made the present invention.
It is a first feature of the present invention that a method of forming a tungsten film on a surface of an object to be processed by a vacuum processing system, said method comprising the steps of: growing tungsten seed crystal grains on the surface of the object to be processed in an atmosphere of a film forming gas containing tungsten atoms; exposing the object to be processed to an atmosphere of a boron-containing gas for a short time; and forming a tungsten film by making the tungsten seed crystal grains grow in an atmosphere of a gas containing a film forming gas containing tungsten atoms, a hydrogen gas and a hydrogen-diluted boron-containing gas.
When forming the tungsten film after growing seed crystal grains of tungsten on the workpiece and exposing the workpiece to the atmosphere of the boron-containing gas for a short time, the film forming gas is supplied to the workpiece in the presence of boron-containing gas and hydrogen gas. Therefore, tungsten crystal grains forming the tungsten film grow large and thereby the tungsten film having a low resistivity can be formed.
It is a second feature of the present invention that a method of forming a tungsten film on a surface of an object to be processed by a vacuum processing system, said method comprising the steps of: growing tungsten seed crystal grains on the surface of the object to be processed in an atmosphere of a film forming gas containing tungsten atoms; and forming a tungsten film by making the tungsten seed crystal grains grow in an atmosphere of a gas containing a film forming gas containing tungsten atoms, hydrogen gas, and a hydrogen-diluted boron-containing gas.
Although the tungsten film forming method according to the second aspect of the present invention omits the boron-exposure process included in the tungsten film forming method according to the first aspect of the present invention, the same method is able to form a tungsten film having a relatively low resistivity.
It is a third feature of the present invention that, when the boron-containing gas is a 5% hydrogen-diluted B2H6 gas, the flow rate of the boron-containing gas is about 0.85% or above of the total flow rate of all the gases. When the boron-containing gas is supplied at such a flow rate, the resistivity is included in a preferable range, the boron-containing gas undergoes self decomposition and boron adsorption and bonding occur on a growth surface in the boron-exposure process.
It is a fourth feature of the present invention that a seed crystal grain layer formed in the seed crystal growing process is 50 nm or below in thickness. The seed crystal grain layer of such a thickness is preferable in view of forming a tungsten film having a low resistivity.
It is a fifth feature of the present invention that the total amount of all the gases supplied every minute in the tungsten film forming process is about 100% or above of the volume of the processing vessel of the vacuum processing system. Supply of the gases at such a flow rate is preferable in view of forming a tungsten film having a low resistivity
It is a sixth feature of the present invention that, the tungsten film forming process achieves simultaneously, for example, both filling up holes formed in the surface of the object to be processed and forming wiring lines.
It is a seventh feature of the present invention that method of forming a tungsten film on a surface of an object to be processed by a vacuum processing system, said method comprising the steps of: growing tungsten seed crystal grains on the surface of the object to be processed in an atmosphere of a film forming gas containing tungsten atoms; exposing the object to be processed to an atmosphere of a boron-containing gas for a short time; and forming a tungsten film by making the tungsten seed crystal grains grow in an atmosphere of a gas containing a film forming gas containing tungsten atoms.
When the tungsten seed crystal grains are processed by boron surface treatment to expose the workpiece to an atmosphere of a boron-containing gas after the formation of the tungsten seed crystal grains and immediately before the formation of the tungsten film, a tungsten film of large tungsten crystal grains having a low resistivity can be formed.
It is an eighth feature of the present invention that, when the boron-containing gas is a 5% hydrogen-diluted B2H6 gas, the flow rate of the boron-containing gas is about 0.85% or above of the total flow rate of all the gases. When the boron-containing gas is supplied at such a flow rate, a tungsten film having a considerably low resistivity can be formed.
It is a ninth feature of the present invention that the tungsten film forming process achieves simultaneously, for example, both filling up holes formed in the surface of the object to be processed and forming wiring lines.